In the prior art, high speed, optimal manufacture of drawn and ironed aluminum beverage cans has been highly developed in conjunction with the optimal properties and microstructure of the input can stock. It has been observed that an optimal distribution of Fe Mn Al.sub.x precipitated phase in Aluminum Association (AA) registered aluminum alloys 3104 and 3004 is important in polishing the dies of the can making equipment in order to prevent excessive generation of fines, galling and wear. In order to achieve these desired metallurgical properties, these types of aluminum alloys have been produced by conventional casting techniques followed by rolling and heat treatment cycles prior to the can making operation.
As an alternative to producing aluminum alloys by conventional casting techniques, a direct casting method has been developed wherein molten aluminum is cast in the form of a metal strip and formed into a coil on a coiler. Generally, in this process molten aluminum is deposited on a moving chill surface from a tundish having an open outlet. An inlet is provided for the flow of molten metal into the tundish from a source of molten metal. The direct casting of the molten aluminum metal onto a chill wheel, preferably a grooved chill wheel, produces a cast aluminum product at a rapid rate. The aluminum cast strip is wound on the coiler in heated form. Drag casting apparatus and methods of this type are described, for example, in
U.S. Pat. Nos. 4,828,012, 4,751,957, 4,896,715, and 4,934,443 and in PCT publications W089/09667, published Oct. 10, 1989, and W090/05604, published May 3, 1990. The disclosures of these patents and PCT publications are hereby specifically incorporated by reference with respect to the methods for the production of aluminum strip and coil formed from molten aluminum or aluminum alloys.
The drag casting process as described above produces a cast product that includes drawbacks preventing its application in deep drawing operations which are generally employed in beverage container making. Due to the rapid cooling of the cast product, the grain structure necessary for efficient drawing operations cannot be obtained. Specifically, intermetallic compounds such as Fe Mn Al.sub.x are not sufficiently large enough or distributed properly within the microstructure to ensure the proper die polishing during the drawing steps. In addition, the top surface of the drag cast product generally has higher levels of porosity than products made using conventional casting techniques and the bottom surfaces of drag cast products include grooves therein as a result of the casting drum texture. Additionally, drag cast products exhibit lower levels of strength, tensile elongation, and ductility than conventionally produced beverage container stock material. Other problems associated with these direct cast products include difficulties in maintaining gauges within tolerances and proper sheet shape or profile.
Given the process' advantages and economic benefits of forming aluminum alloys using direct drag casting techniques rather than conventional casting and the vast tonnages of aluminum alloy beverage container stock material produced, a need has developed to produce aluminum alloy beverage container stock material by methods having reduced unit costs.
In response to this need, it has been discovered that aluminum alloy beverage container stock material may be produced by drag casting techniques when the drag cast product is coated with a polymeric material prior to subsequent drawing processes. The polymeric coating overcomes problems associated with the use of drag cast aluminum alloy material for aluminum beverage container stock material. The polymeric coating separates the aluminum alloy from the drawing tools such that the metallurgical structure of the drag cast product is less important to ensure product quality. Furthermore, the polymeric coating provides a lubricating media which minimizes the criticality of the aluminum alloy sheet gauge and flatness and ameliorates issues of ductility.
In the prior art, the use of organic coatings for aluminum beverage containers is known. The method of applying known organic coatings such as polyethylenes, polypropylenes or polyolefins includes cleaning and pretreating the aluminum sheet and subsequently coating the aluminum sheet in continuous operations. The organic coatings may be applied on one or both sides with one or two coats per side. In a two coat system, the first coat may be designed for good adhesion to the aluminum alloy sheet while a second coat is designed to improve effectiveness of printing inks thereon. U.S. Pat. No. 4,945,008 to Hayes et al. discloses a method of laminating a polymer material to aluminum alloy can stock material as well as products produced by the process thereof. In this process, the aluminum alloy sheet is heated and the laminate is applied thereto using lamination rolls. The laminated sheet material may then be further heated, quenched and dried with a blast of air.
None of the above-mentioned prior art documents teaches or suggests how to produce a polymer laminated drag cast aluminum alloy sheet material suitable for use in drawing operations. Furthermore, none of the prior art cited above teaches or suggests a drag cast aluminum alloy product that is suitable for use as beverage container stock material because of the provision of a polymer material on at least one surface thereof to overcome metallurgical deficiencies in the product of the drag cast aluminum alloy sheet material relating to drawing operations.